1. Field of the Invention
The present invention relates to a lens barrel adapted for a camera or the like, and more particularly to a so-called differential-type lens barrel arranged to permit reduction in diameter of the whole lens barrel by reducing the thickness of a differential tube.
2. Description of Related Art
Zoom lens barrels of the kind having a cam pin, a cam groove and a rectilinear motion guide member have heretofore been variously arranged.
Meanwhile, a lens barrel disclosed in Japanese Laid-Open Patent Application No. HEI 3-209445 is arranged to use a helicoid in place of the cam pin and the cam groove. In the case of this lens barrel, a front lens unit is provided with a male helicoid and a rear lens unit is provided with a cam pin, while a cam tube is provided with a female helicoid and a cam groove for guiding and holding the two lens units. These parts are arranged to permit the lens barrel to be drawn out by using the cam tube.
In this lens barrel, the front lens unit which is arranged to be guided by the helicoid mechanism is guided through a single lead screw. The front lens unit, therefore, linearly moves while the cam tube is rotating. However, if the cam groove of the cam tube is formed to have such a locus that enables the rear lens unit being guide by the cam groove to satisfy an optical condition and a condition for moving into a stowage state, the front lens unit makes a nonlinear motion in relation to the rotation of the cam tube.
In this instance, the cam groove is formed to cross a part of the female helicoid. Therefore, while the cam groove is in a continuous shape, the female helicoid is partly noncontinuous. In other words, in this state, the cam groove which is formed to satisfy the optical loci required for the front and rear lens units and the loci necessary for moving them to their stowage positions interferes with the female helicoid within the cam tube.
To solve this problem, therefore, the lens barrel disclosed in the above-cited Japanese laid-open patent application is arranged to have the positions of the helicoid and the cam groove within the cam tube deviate stepwise from each other in the direction of diameter. This arrangement effectively avoids the above-stated interference, thereby enabling both the helicoid and the cam groove to fulfill their respective functions.
In another lens barrel disclosed in Japanese Laid-Open Patent Application No. HEI 4-347810, a rotating tube is arranged to guide only a front lens unit by helicoid coupling. A cam tube which is disposed inside of the front lens unit is arranged to be rotated by the force of a moving amount of the front lens unit in the direction of an optical axis. The rotation of the cam tube causes a rear lens unit to make a nonlinear motion in relation to the rotation of the rotating tube.
In the case of this lens barrel, a lens barrier is opened and closed by making a zero lead state at a part of the helicoid which is guiding with a single lead (at the time of stowing the lens barrel), i.e., by bringing about a state in which the front lens unit does not move in relation to the rotation of the rotating tube.
In this lens barrel, a helicoid and a cam groove are arranged to have a lead varying at a part. However, since the helicoid and the cam groove are formed in different tubes, they do not interfere with each other, so that the tubes can be arranged without taking heed to each other.
Further, there are many known differential-type zoom lens barrels having a plurality of tubes arranged in such a way as to make the total length of the lens barrel shorter in a stowage state.
The structural arrangement of differential-type zoom lens barrels is advantageous for making the length in the stowage state of the lens barrel shorter. However, since a plurality of tubes are arranged to be in fitting engagement, the diameter of the whole lens barrel becomes large and the projected area of the lens barrel as viewed from the front side of the camera inevitably becomes large. The diameter of the lens barrel can be prevented from becoming large, by decreasing the thickness (dimension in the redial direction) of each of the component tubes. However, the decreased thickness lowers the rigidity and strength of the whole structural arrangement.
With respect to the thickness of a cam tube in a differential-type lens barrel, the thickness determining elements of the cam tube of the lens barrel disclosed in Japanese Laid-Open Patent Application No. HEI 3-209445 include, in sequence from outside, the following three:
(1) A part which is a base of the whole tube.
(2) A cam groove for cam coupling with a cam pin.
(3) A helicoid screw for helicoid coupling.
If the arrangement of the cam tube is changed to perform the drawing-out action totally by cam coupling by rescinding the drawing out by the helicoid coupling, the front lens unit can be nonlinearly moved in relation to the rotation of the cam tube, similarly to the rear lens unit, to avoid any inference taking place within the cam tube. By such modification, the three thickness determining elements can be reduced to the elements (1) and (2) to permit reduction in thickness of the cam tube.
However, compared with the case where a helicoid is used, the number of cam pins necessary for the cam pin arrangement increases. Besides, the cam coupling which has three contact points is weaker in coupling strength than the face-engaging arrangement of helicoid coupling.
As regards the lens barrel disclosed in Japanese Laid-Open Patent Application No. HEI 4-347810, the lens barrel has a helicoid coupling part and a cam coupling part formed in different tubes. In respect of thickness, however, the lens barrel necessitates the thickness determining element (1) at two parts. Therefore, the arrangement of the lens barrel inevitably causes an increase in the diameter thereof.
It is an object of the invention to provide a lens barrel which is arranged to solve the problems of prior art described in the foregoing.
To attain the above object, in accordance with an aspect of the invention, there is provided a lens barrel, which comprises a cam tube having a female helicoid and a cam formed in an inner wall thereof, a first tube which holds a lens and has a male helicoid formed on an outer wall thereof to engage the female helicoid, the first tube being arranged to be moved in an optical axis direction by rotation of the cam tube, and a lens holding member which holds a lens and has a cam pin provided thereon for engaging the cam of the cam tube, the lens holding member being arranged to be moved in the optical axis direction by a cam action of rotation of the cam tube, wherein the female helicoid and the cam of the cam tube respectively have loci which are continuous and uninterrupted without interfering with each other.
Further, in the lens barrel, the female helicoid is a composite female helicoid composed of a first female helicoid part and a second female helicoid part which is formed continuously from the first female helicoid part and has a lead different from a lead of the first female helicoid part, and the male helicoid of the first tube is a composite male helicoid composed of first and second male helicoid parts which are capable of face-engaging the first and second female helicoid parts of the female helicoid.
By virtue of the above-stated arrangement, the cam tube can be compactly formed to satisfy the necessary conditions for both helicoid coupling and cam coupling without increasing the thickness of the cam tube and thus without increasing the diameter of the whole lens barrel.
In the lens barrel, the cam of the cam tube is preferably formed in a rugged, protruding-and-recessed shape alternately and continuously having a part at which the lens holding member is caused to be moved, for example, in the frontward direction of the optical axis and another part at which the lens holding member is caused to be moved in the rearward direction of the optical axis. A part of the cam tube where the first and second female helicoid parts continue to each other is formed in the neighborhood of a part of the cam where the cam has a recessed shape on the side of the female helicoid, so that the cam and the female helicoid can be arranged close to each other without the fear of the interference of them with each other. This arrangement effectively reduces the size of the cam tube in the optical axis direction.
In the lens barrel, the first female helicoid part of the composite female helicoid formed in the cam tube is preferably arranged to move the first tube in such a way as to carry out both a magnification varying action and a focusing action. The second female helicoid part is preferably arranged to move the first tube for an action other than the magnification varying action and the focusing action, such as a lens barrel stowing action, in such a way as to ensure that the lens barrel can be smoothly drawn out through the helicoid coupling in an ordinary zoom driving operation.
Further, in the composite male helicoid formed in the first tube, the first male helicoid part is preferably arranged to have a longer engaging length than that of the second male helicoid part, in such a way as to make the lens barrel have a sufficient strength against external forces in its drawn-out state.
The above and further objects and features of the invention will become apparent from the following detailed description of a preferred embodiment thereof taken in connection with the accompanying drawings.